In a conventional air-fuel ratio sensor control apparatus, a voltage is applied to a cell of an air-fuel ratio sensor and a current flowing in the cell is detected as a sensor current, which indicates an air-fuel ratio. In an air-fuel ratio sensor control apparatus disclosed in JP 2015-148471A (US 2015/0219590 A1), a DC voltage applied to a sensor is varied in accordance with a sensor current.
The cell of the air-fuel ratio sensor has a capacitive component (that is, electrostatic capacitance). For this reason, in case of digitally controlling the voltage applied to the cell, a step change of the voltage causes a current flow in the cell through the capacitive component. In case that the current flowing through the capacitive component of the cell (hereinafter referred to as a capacitive current) is detected as a part of the sensor current, accuracy of air-fuel ratio detection is lowered.